Maintenance; Test Procedures - HP LOGIC PULSER 546A Operating And Service Manual

Page 20 Model 546A
"0''
OVERCURRENT CYCLE. When output occurs, transistor Q2 saturates and the sense circuit in
U1 (pin 16) begins measuring voltage on the tip through resistor R2 and capacitor C3. With transistor
Q2 conducting heavy current from the load through the tip, capacitor C2 develops a voltage charge.
When this voltage reaches approximately 0.8 volts (relative to ground) pulse shutdown occurs in the
U1 sensing circuit and transistor Q2 turns off.
"1" OVERCURRENT CYCLE. The "1" cycle is similar to the "0" cycle except that transistor Q1
saturates, the direction of current flow is reversed and the shutdown threshold voltage is referenced
to V+ (set to approximately V+ minus 1.5 volts). In addition, capacitor C2 supplies most of the output
current through transistor Q1 thus reducing current surges on the V+ line.
In either overcurrent cycle the fast turn on and turn off of transistors Q1 and Q2 provides sharp pulse
edge transitions through logic threshold transition regions. The "1" or "0" overcurrent output cycle
is repeated each time a high current pulse is outputted.
MAINTENANCE
The following paragraphs provide test procedures, probe disassembly and assembly and parts re-
placement information.
Test Procedures
The following performance tests ensure that the Pulser is operating properly. Test equipment re-
quired is listed in Table 4. Test equipment having equivalent characteristics may be substituted.